This invention relates to hydraulic percussive machines of the kind which can be used as rock drilling machines.
Hydraulic machines are known which work on the principle of differential piston areas operating in two working chambers. In such a case one chamber, usually the return chamber, is constantly at systems pressure, while the drive chamber exposed to the larger piston area is alternatively at systems pressure or connected to tank.
In one kind of machine a shuttle valve also having differential end areas is moved to an fro to connect the drive chamber to systems pressure and to tank in turn. Timing lands on the piston cover and uncover ports in the wall of the cylinder defining the chambers. Since the timing lands would wreak havoc with any seals intermediate the two chambers, truly effective sealing is not possible. One has to rely on the closeness of the fit of the piston in the cylinder.